Materials based on group IV chalcogenides exhibit extensive technologically important properties. Its unusual chemical bonding and off-centering of in-layer sublattices could cause chemical polarity and weakly broken symmetry, making optical field controlling feasible. Here, we fabricated large-area SnS multilayer films and observed unexpected strong SHG response at 1030 nm. The appreciable SHG intensities were obtained with an independence on layer, which is opposite to the generation principle of overall nonzero dipole moment only in odd-layer material. Taking GaAs for reference, the second-order susceptibility was estimated to be 7.25 pm/V enhanced by mixed-chemical bonding polarity. Further polarization-dependent SHG intensity confirmed the crystalline orientation of SnS films. The results imply surface inversion symmetry broken and nonzero polarization field modified by metavalent bonding should be the origin of SHG responses. Our observations establish multilayer SnS as a promising nonlinear material, and will guide in design of IV chalcogenides with improved optics and photonics properties for the potential applications.

For example, ultrathin flexible electrode arrays could be installed on epidermis [16] or organ surfaces [17] for real-time monitoring of human health that cannot be realized by traditional rigid electronic devices. Thus, new functional materials of flexible electronics that not only meet the demand of high performance but also have good stability and durability under reiterative bending, folding, or stretching are urgently pursued.2D transition metal dichalcogenides (2D TMDs) are a rising star in the family of 2D layered materials since their outstanding electrical and optical properties, such like tunable bandgaps, [18] high absorbance, [19] quick carrier generation, and migration. [20] More importantly, 2D TMDs materials are featured by strong covalent intralayer bonds and weak van der Waals (vdW) interlayer bonding, which make them a promising candidate to build atomicscale or nanoscale Lego devices as well as flexible electronics applications. [21][22][23] Studies have shown that covalent bond can tolerate enormously higher strains (up to 40%) than ionic bonded bulk forms (≈18%), which can enable 2D TMDs sustain the deformation over 10% before rupture, [24] while the fracture value of typical bulk semiconductors is usually less than 1%. [25] Besides, according to the dependence of the stiffness (k) on cross-section (A) and length (L) expressed as k = E × A/L, 2D TMDs are expected to have excellent flexibility with a very low k due to their ultrathin thicknesses. [26] Layered molybdenum disulfide (MoS 2 ), as a semiconductor material with an indirect bandgap of 1.2-1.8 eV (direct bandgap in monolayer materials), has been employed in lots of photoelectronic devices. Up to now, substantial developments and progresses concerned have been witnessed, especially on flexible photodetectors owing to its terrific mechanical flexibility with 11% stretch. [27] Its reducible atomic thickness greatly facilitates the design and construct for bending, crimping, and folding, whereas it comes at the expense of excellent optoelectronic properties. [28] The carrier mobilities of MoS 2 on flexible substrates range from 4.7 to 45 cm 2 V −1 s −1 , [29] while that on Si substrate is up to 480 cm 2 V −1 s −1 . [30] The high variation should be caused by introducing impurities and creating sources of extrinsic scattering in physical contacts, which reduces the mean-free path of the carriers. [31] The latest researches show that, the synergetic integration of two layered materials is a promising strategy to overcome the shortages of individual Flexible electronics is one of the hotspots of interdisciplinary research and can promote disruptive technology for post-Moore applications in the field of biomedical, electronic skin, wearable devices, etc. 2D materials have triggered great interest in flexible electronic devices since they have tunable bandgaps, excellent mechanical flexibility, good chemical stability, and outstanding optical properties. Their reducible atomic thickness greatly facilitates the design and construct for bending,...

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Light absorption and radiation process is one of the fundamental processes in optical science and engineering. Materials with perfect absorption properties play an important role in numerous optical applications. Following the meteoric rise of MoS2 material, global opportunities and challenges coexisted since its extremely weak light-matter interaction capability beyond its energy band. In this work, we designed a kind of sandwich resonance structure and expressed MoS2 first as a perfect absorber in infrared spectrum that should be transparent according to optical band theory. The infrared absorption properties of W or Au/MoS2/Au models in 800–2400 nm were systematic simulated. By optimizing the structural parameters, the resonant wavelength of perfect absorption can be modulated from 830 to 1700 nm with angle-insensitivity and polar-independence. Moreover, we discovered the bandwidth of absorption exceeding 50% of W-top model reaches 500 nm, while that of Au-top model is less than 100 nm, indicating the top metal material used have a great influence on the resonance absorption spectrum. Our works provide a practical route to enhance and manipulate the light-matter interactions of low-dimensional materials beyond their own band gaps, which will be critical in the future design and implementation of optoelectronic devices and systems.